Making a karyotype is a multi-step laboratory process that involves collecting living cells, coaxing them to divide, freezing them at the right moment, and then staining and photographing their chromosomes so they can be sorted into numbered pairs. The full process typically takes 1 to 4 weeks depending on the sample type, though rush orders can deliver preliminary results in about 3 business days. Here’s how each step works, from sample collection to the final arranged image.
Why Karyotypes Are Made
A karyotype gives a complete picture of someone’s chromosomes: all 46 of them, arranged by size and shape, so that missing, extra, or structurally altered chromosomes become visible. Clinicians order karyotypes for a range of reasons. Newborns or children with signs of a genetic condition like Down syndrome (an extra chromosome 21) or Turner syndrome (a missing X chromosome) often need one. Adults may get karyotyped when investigating unexplained infertility, recurrent miscarriages, or stillbirths. And certain cancers, particularly leukemia and lymphoma, involve chromosome changes that affect treatment decisions, so karyotyping is part of the diagnostic workup.
Collecting the Right Sample
Karyotyping requires living cells that can be grown in a lab, so the sample has to be handled carefully. The most common source is a simple blood draw. Adults provide 5 to 7 milliliters in a heparinized tube (the green-top tube), while infants need only about 2 to 2.5 milliliters. Bone marrow aspirates, used for cancer-related karyotyping, also require roughly 2 to 2.5 milliliters. For prenatal testing, 18 to 20 milliliters of amniotic fluid is collected during amniocentesis.
The cells must stay alive. If the sample can’t be delivered to the lab immediately, it’s refrigerated to preserve cell viability. Post-mortem samples need to be collected within one hour and kept at room temperature.
Growing Cells in Culture
Once the sample arrives, lab technicians place the cells into a nutrient-rich culture medium and incubate them at body temperature. The goal is to get as many cells as possible actively dividing, because chromosomes are only visible during cell division. Blood samples contain white blood cells (specifically lymphocytes), which are stimulated to divide using a growth-promoting chemical. Amniotic fluid cells and bone marrow cells divide more slowly, which is one reason prenatal and cancer karyotypes take longer to complete.
The culture period varies. Blood cultures typically grow for about 72 hours. Amniotic fluid cultures can take one to two weeks before enough dividing cells are available.
Arresting Cells at the Right Moment
Chromosomes are only clearly visible during one brief phase of cell division called metaphase, when they condense into their familiar X-shaped forms. To catch cells at exactly this stage, the lab adds a chemical called colcemid (also known as demecolcine) to the culture. Colcemid works by blocking the formation of the spindle fibers that normally pull chromosomes apart. Without those fibers, cells stall at metaphase, and the chromosomes stay condensed and separated, ideal for photographing.
The timing of this step matters. Too little exposure and not enough cells are arrested. Too much and the chromosomes over-condense, making them stubby and hard to analyze.
Swelling and Spreading the Chromosomes
After the cells are arrested, technicians remove the culture medium and replace it with a hypotonic solution, typically potassium chloride dissolved in water. This solution has a lower concentration of dissolved particles than the inside of the cell, so water rushes in by osmosis. The cells swell dramatically, which pushes the chromosomes apart from one another. Without this step, the chromosomes would clump together in a tangled mass.
Next, the swollen cells are preserved with a chemical fixative (usually a mixture of methanol and acetic acid) that hardens the cell structure and prepares it for slide-making. The fixed cell suspension is then dropped onto a glass microscope slide from a specific height. When the droplet hits the slide, the cells burst open and the chromosomes spread out flat, a preparation called a “metaphase spread.”
Staining to Reveal Banding Patterns
At this point, the chromosomes on the slide are transparent and nearly invisible. Staining is what makes them identifiable. The gold standard technique is called G-banding, short for Giemsa banding. The slide is first briefly treated with trypsin, an enzyme that partially digests proteins along the chromosome surface. Then it’s stained with Giemsa dye.
The result is a pattern of alternating dark and light horizontal bands along each chromosome. Every chromosome has a unique banding pattern, like a barcode, that allows technologists to tell chromosome 1 from chromosome 2, or spot a piece of chromosome 9 that’s been accidentally swapped onto chromosome 22. The dark bands generally correspond to regions of tightly packed, gene-poor DNA, while the lighter bands tend to contain more active genes. A skilled technologist can typically resolve 400 to 550 individual bands across the full set of human chromosomes.
Photographing and Arranging the Karyogram
Once the chromosomes are stained, a technologist scans the slide under a microscope to find well-spread metaphase cells where all 46 chromosomes are clearly separated and well-banded. The microscope is connected to a digital camera that captures a high-resolution image of each spread.
From there, individual chromosomes are digitally cut out of the image and arranged into a standardized layout called a karyogram. Chromosomes are sorted into 23 pairs, numbered 1 through 22 by size (largest to smallest), plus the two sex chromosomes (XX or XY). Within each pair, the chromosomes are oriented with their short arm on top and long arm on the bottom, aligned at the centromere (the pinched midpoint).
This arrangement follows an international naming system called the ISCN (International System for Human Cytogenomic Nomenclature), most recently updated in 2024. The ISCN provides standardized shorthand for describing what the karyotype shows. A normal male karyotype, for example, is written as 46,XY. A female with Down syndrome would be 47,XX,+21, indicating 47 total chromosomes with an extra copy of chromosome 21.
How Software and AI Speed Up the Process
Traditionally, the entire process of scanning slides, selecting good spreads, and arranging chromosomes was done by hand, a painstaking task that could take 30 minutes or more per cell. Modern labs now use AI-guided karyotyping software that automates much of this work. These systems use deep learning algorithms, specifically convolutional neural networks, to identify individual chromosomes in a metaphase image, trace their boundaries, and classify them into the correct pairs.
Several commercial platforms are available. Some use real-time processing to present hundreds of preliminary karyotyped images for a technologist to review, rather than requiring manual arrangement from scratch. The technologist still makes the final call, checking for subtle abnormalities the software might miss, but the automation dramatically reduces hands-on time and improves consistency across labs.
How Long Results Take
For a standard blood karyotype, expect results within about 2 to 4 weeks. The culture period accounts for a few days, but the analysis, quality checks, and reporting add time. Rush orders can produce a preliminary result within 3 business days, with a finalized report following within about 14 days. Prenatal karyotypes from amniotic fluid generally take longer because the cells grow more slowly in culture.
What Karyotyping Can and Cannot Detect
Karyotyping excels at detecting large-scale chromosome problems: extra or missing whole chromosomes (like trisomy 21), large deletions or duplications, and structural rearrangements where pieces of chromosomes have swapped places. It’s also the only routine test that can detect balanced translocations, where chromosome segments trade places without any DNA being gained or lost. These balanced rearrangements affect roughly 1 in 1,000 prenatal samples and are invisible to newer array-based tests.
The main limitation is resolution. Karyotyping can only detect changes large enough to see under a microscope, generally deletions or duplications spanning several million DNA base pairs. Smaller changes slip through. In a large study of prenatal samples published in the New England Journal of Medicine, chromosomal microarray analysis found clinically significant abnormalities in 6% of fetuses with structural anomalies on ultrasound that had been missed by standard karyotyping. Among women tested solely because of advanced maternal age, microarray caught additional findings in about 1.7% of cases with normal karyotypes.
On the other hand, microarray cannot detect balanced translocations or triploidy (a condition where the cell has three complete sets of chromosomes instead of two). In that same study, none of the 17 triploid samples were identified by microarray. This is why karyotyping remains a core tool in cytogenetics despite the availability of higher-resolution molecular tests. The two approaches complement each other, catching different types of abnormalities.